Selective catalytic reduction (SCR) systems may be used in a vehicle to facilitate reduction of engine output NOx by a reductant, such as urea or ammonia. An SCR system involves injecting the reductant upstream of an SCR catalyst where the reductant, or reductant products, can react with NOx to create byproducts such as nitrogen and water. An example NOx reduction system having a first and second catalyst bed in series is described in U.S. Patent Application US 2005/0284134A1 (Radhamohan et al)
During operation, SCR systems may experience various forms of degradation, such as contamination, thermal degradation, etc. However, in the example of multiple SCR regions, different levels of degradation of the different SCR regions may be trigger emission levels to rise above a threshold.
The inventors herein have recognized the above issues and developed various approaches in response. In one example, methods and systems for diagnosing a first SCR region and/or a second SCR region positioned in series are described. The method may comprise indicating degradation based on a first SCR region performance during a first condition; and indicating degradation based on a second SCR region performance during a second condition, the first condition different than the second condition.
For example, the method may differentiate the performance levels of the different SCR regions, and thus apply different threshold degradation levels to the different SCR regions. In this way, diagnostics can take into account the differing effects of degradation among the various SCR regions on the overall emission levels.
In one example, the first condition may include an engine cold start when a temperature of the first SCR region is below a high threshold and a temperature of the second SCR region is below a low threshold. Further, the second condition may include a DPF regeneration when the temperature of the first SCR region is above the high threshold and the temperature of the second SCR region is above the low threshold. By performing diagnostics when a temperature of one of the SCR regions is within a particular range, the performance of the remaining SCR region(s) can be evaluated while maintaining NOx conversion efficiency and reductant slip risk in reasonable ranges, and further isolating measurable performance to one or more particular regions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.